Complete cDNA and genomic sequence encoding a flooding-responsive gene from maize (Zea mays L.) homologous to xyloglucan endotransglycosylase.

The vast majority of identified flooding-responsive genes encode enzymes of Glc-P metabolism, which function to allow limited energy production in the face of limited oxygen supply (Sachs, 1994). However, in addition to the induction of these genes, flooded plants also undergo structural changes, such as formation of intercellular air spaces, which may help in survival by facilitating gas exchange between submerged and aerated plant parts (Drew et al., 1979). These structural modifications have not been addressed at the molecular level. Here we report the isolation of a complete cDNA (MZEXETHOM) and genomic sequence (g2005) encoding a flooding-responsive gene that is homologous to XET and related glucanases (Table I). XET is a recently discovered enzyme that is proposed to play a role in wall loosening during cell expansion and fruit softening ( e g Nishitani and Tominaga, 1992; Redgwell and Fry, 1993). The activity of XET is that of transglycosylation: the enzyme catalyzes cutting and rejoining of xyloglucans, and thus allows molecular grafting between polysaccharide chains that crosslink cellulose microfibrils (Fry et al., 1992; Nishitani and Tominaga, 1992). In accordance with its proposed role, increased XET activity was correlated with growth of cells in culture (Hetherington and Fry, 1993) and with altered wall rheology during water deficit (Spollen et al., 1993). In addition, increased XET activity was also correlated with fruit ripening, which results from enzymatic degradation of the cell wall, leading to loss of structural integrity. In this case, XET is proposed to loosen the cell wall prior to degradation by other enzymes (Redgwell and Fry, 1993). Thus, g2005 is likely to encode a flooding-responsive form of XET or a related glucanase, which may be involved in cell wall metabolism during processes leading to structural modifications. The deduced amino acid sequence of MZEXETHOM exhibits homology to XET sequences from a range of species